This invention relates to the venting of lead acid storage batteries. More particularly, it relates to a ceramic body which is placed in the filling cap or the cover of a lead acid storage battery and serves to permit the cell gases to vent through the ceramic stone while at the same time effectively resisting any ingress of flame caused by the ignition of cell vent gases. The ceramic body is designed to effectively retard flame propagation while preventing the accumulation of large volumes of gases in the battery.
Ceramic materials have been utilized in conjunction with storage batteries of the lead acid type so as to prevent an external spark or flame from causing an explosion inside the battery. For example, in U.S. Pat. No. 2,341,382 such an arrangement is described employing a loose ceramic material. In U.S. Pat. Nos. 2,471,585 and 3,630,788 a combined filling and venting device employs a ceramic material in the form of a tubular structure. It is now proposed to utilize flat plate-like slabs of ceramic material in the gang type vent cap as well as a single vent cap. Such a device is described in U.S. patent application Ser. No. 354,514 filed Apr. 26, 1973, now abandoned, which was refiled as U.S. patent application Ser. No. 423,055 filed Dec. 10, 1973, now U.S. Pat. No. 3,879,227, both of which are commonly assigned.
The problem with prior ceramic materials utilized in battery venting devices is the maintenance of open porosity so that the material will adequately vent the battery yet will serve as a barrier to a flame. Prior to this invention, particle distributions were not uniform which is very important when a small size ceramic disc or platelet is to be employed in a battery vent. Other problems with available ceramic materials reside in excessive wear in the dies utilized to stamp the green ceramic material for example, fused alumina, prior to their being fired in a kiln. In order to be effective as a flame barrier as well as a venting material, the ceramic must have a very critical particle size range, be able to withstand normal handling as it is bulk shipped, be acid and heat resistant and not collapse during pressing so as to expose particles which are not wetted with bonding agent.
It is an object of the present invention to provide a novel porous ceramic material which very effectively serves to vent explosive gases from storage batteries in a manner that any external ignition is impeded from being transmitted into the battery casing. It is another object of this invention to provide a porous ceramic material which exhibits a narrow controlled range of back pressure under given gas flow conditions. It is still another object of this invention to provide a novel unfired ceramic material which reduces the wearing action on the pressing tools during its pelletizing. It is yet another object of this invention to provide a fired ceramic material which is acid resistant, heat resistant and also has good strength after being fired so that it can be handled in bulk shipments and assembling equipment without cracking or chipping.